1. Technical Field
The present disclosure generally relates to integrated photodetectors, for example, photodetectors of an image sensor. More specifically, the present disclosure relates to the protection—or hardening—of such photodetectors against ionizing radiations.
2. Discussion of the Related Art
Image sensors may comprise photosensitive sites and transistors formed in a semiconductor substrate, for example, silicon. More specifically, such image sensors comprise a pixel array, each pixel comprising at least one photodiode comprising a P-N junction formed in the substrate. Photons reaching the pixel may cause the forming of electron/hole pairs in the substrate. The electron/hole pairs may form in the photodiode depletion area. The electric field present in the depletion area then directs electrons towards the N-type doped region and holes towards the P-type doped region. The electron/hole pairs may form in the substrate in a P-type or N-type doped region. When an electron/hole pair forms at a shorter distance from the depletion area than the minority carrier diffusion distance, the minority carrier (electron or hole) may diffuse all the way to the depletion area. The electric field present in the depletion area then directs the minority carrier towards the P-type or N-type region where it is a majority carrier. The collection of electrons and holes by the photodiode appears as a measurable variation of the voltage across the photodiode.
The quantum efficiency of the pixel corresponds to the ratio of the number of electrons that can be collected by the photodiode to the number of photons reaching the pixel. It is desirable to have the highest possible quantum efficiency.
The dark current of the pixel corresponds to the signal provided by the photodiode in the absence of any lighting. It results from the forming of electron/hole pairs in the pixel in the absence of any lighting, which electrons and holes may be collected by the photodiode. Thermal agitation generally causes the forming of electron/hole pairs in the substrate. The presence of defects at the interface between the semiconductor substrate and a portion of an insulating material increases the electron/hole pair forming speed in the substrate at the level of this interface, especially in a depletion area. Such is especially the case at the interface between the semiconductor substrate and the insulating layer covering the substrate or at the interface between the semiconductor substrate and insulating trenches formed in the substrate to insulate doped regions.
Image sensors may be submitted to an ionizing radiation, for example, a gamma radiation, in particular when they are used for space applications.
The dark current of the pixel may vary under the effect of the ionizing radiation. Two phenomena may cause a variation of the dark current. First, the state of the interfaces between the insulating portions and the semiconductor substrate may damage under the action of the ionizing radiation, which increases the electron/hole forming speed at these interfaces. Second, the ionizing radiation may cause the forming of positive charges in the insulating portions of the pixel. By electrostatic effect, such positive charges push back the holes present at the interface of the P-type doped regions. This causes a local increase of the dimensions of the depletion area in the P-type doped region close to the interface with the insulating portion, and thus an increase of the number of electron/hole pairs formed in the absence of any lighting at the interface between the substrate and the insulating portion in the depletion area.
The pixel hardening especially aims at decreasing the variation of the dark current of the pixel in the presence of an ionizing radiation.
The pixel of an image sensor may comprise the three following electronic components: the photodiode, a charge reading transistor, a reset transistor, and a selection transistor. The photodiode is in charge of collecting and storing the charges photogenerated in the pixel (for example, electrons). The photodiode is connected to the charge reading transistor (follower-assembled transistor) and to the reset transistor. The line selection transistor allows a sequential line-by-line reading.
For many image sensors, the substrate is doped with a first conductivity type and the photodiode of the pixel is obtained by forming, at the substrate surface, a doped region of the second conductivity type. The doped region is laterally insulated by an insulating trench formed in the substrate. The quantum efficiency of the photodiode may be increased by increasing the surface of the P-N junction, that is, by a lateral extension of the doped region. This however increases the insulating trench dimensions. The dark current thus increases, as well as the sensitivity of the dark current to ionizing radiations.
There thus is a need for a photodiode simultaneously having a high quantum efficiency and a decreased sensitivity of the dark current to ionizing radiations.